new light on old sites


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Geophysical Survey Results

Data from the geophysical survey was downloaded onto a computer during our lunch break and at the end of the day. This allowed us to begin to ‘see’ the site as it unfolded very quickly, as well as allowing us to identify any problems with our survey technique, and in the case of any technical difficulties would could limit the amount of data loss. I for one, couldn’t wait to download the data! This blog post will briefly describe the process we undertook after the data was downloaded.

Raw Data

fieldbscrnshot

Field B showing grid (raw data)

Survey data downloaded from the magnetometers is considered ‘raw’. The data for each grid is stored separately and has to be compiled using software. Computer programmes do not know how to put each grid together, so you have to keep accurate notes when surveying!

We used GEOPLOT 3.0 to work with our data. Because we ended up grid-ing out each of the fields separately, we created compiled images for each individual field. That way, we could take the fences into account.

Data Processing

Once the grids for each field were compiled, we then had to ‘process’ the data. Processing corrects for the slight (but unavoidable) inaccuracies caused by the field slope, different gaits from different surveyors, and even very slight inaccuracies in the grid layout. It also helps to smooth out some of the data. Then the files are exported from GEOPLOT 3.0 and imported into GIS software (I used QGIS for this project). You can see a comparison of the raw data and the processed data below.

SWALL14 raw data

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Raw Data)

 

SWALL14 processed data

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Processed Data)

Interpretation

Based on the fully processed data and our understanding of the site from visits and the 1920s excavation report, we then began to interpret the survey.

Field A

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Field Names)

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Field Names)

Despite the visible earthwork in Field A, which is only faintly visible in the geophysical survey results (marked in blue), no other features of archaeological interest were visible. Dr Clay described this ring earthwork as an amphitheatre or ‘circus’ and considered it to be part of the settlement enclosure visible in Field B. It is unfortunate that the road that separates the two sites cuts through anything that may have indicated the relationship between the two features. However, given the other known dated evidence for both prehistoric and historic features in the immediate vicinity, there is no reason for this earthwork to be directly related to the Iron Age activity.

Field B, C, Ca

The linear features forming the ditch enclosure (marked in green) is particularly strong to the west (were it is visible on the surface), but also on the eastern side. The eastern portion of the ditch visible in field Ca may continue into the south-eastern corner of field C. It is possible that originally both ditches were connected along the southern side, but the road later cut through the site. Another, disconnected, linear feature far to the west of the enclosure ditch is also faintly visible on the survey results.

A number of roughly circular negative features (marked in red) are visible in the area that appears to be the inside of the enclosure. Many of these are likely to be the pits that Clay previously excavated, but it is not possible to exactly identify each pit.

There also seem to be several possible curvilinear features that may indicate round-structures (marked in purple). Clay was unaware that Iron Age people lived in roundhouses at the time of his excavations and instead believed that they lived in pits, so he didn’t know to look for evidence of roundhouses.

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Interpretation)

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Interpretation: Green=Visible Ditches, Blue=Visible Banks, Red=Negative Features, Purple=Ring?)

A full report with technical details about the survey is available here. 


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Geophysics Methodology 101 – Part One

There were two major questions we needed to consider when it came to tackling geophysics for Swallowcliffe:

  1. How do you pick a place to do geophysics?
  2. How do you actually do geophysics?

We’ll walk through each of these questions and really break them down.

Question #1 – How do you pick a place to do geophysics?

As you can see from our previous post on the site’s location, the area we’re looking at is pretty large. English Heritage scheduled an area covering three separate fields:

Swallowcliffe_Down_1945_Field names

Dr. Clay’s 1920s excavation of Swallowcliffe Down took place in an area that overlaps Fields B and C, and parts of Field A were also mapped and possibly explored (the original excavation can be quite unclear sometimes!). When Elizabeth and I made our initial site visit in Spring 2014, we found visual evidence of the site in the lower right-hand corner of Field B:

OLYMPUS DIGITAL CAMERA

It’s difficult to see, but there’s a slight, semi-circular ridge a few meters beyond the fenceline. English Heritage also recorded a series of undated dyke systems at the opposite end of Field B. We also found a raised earthwork in the upper right-hand corner of Field A:

OLYMPUS DIGITAL CAMERA

For these reasons, we decided to prioritize the lower right-hand corner of Field B, all of Field C, and as much of Field A as possible (focusing on the upper right-hand corner). We wanted to survey the dyke systems on the left-hand side of Field B as well, but there were several issues with that:

Swallowcliffebiggrid

We’ll come back to this grid in Question #2, but for now, I would like to point out that each yellow square in the photo above corresponds to a 30 by 30 meter grid square. Therefore, Field B covers an area of about 93,600 square meters. Our team would only consist of six members working two machines in teams of three, and we would only have a week to complete the geophysical survey.

This is why we had to prioritize the specific areas that were mapped in Dr. Clay’s original report (the areas marked in red). We promised ourselves we would survey as much of the center and left side of Field B as possible, depending on how quickly we would get through the area of the site itself and how the data looked. While it would be great to find new things outside the site, we wanted to find the things Dr. Clay missed within the site – specifically, Iron Age roundhouses and four-post structures.

So, now you know how we picked the site. Coming up next: how to do geophysics!


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Swallowcliffe Down Project

Hello, and welcome to the ‘New light on old sites’ blog where we will chronicle our fieldwork adventures. The name of our project reflects our interest in re-evaluating sites that were excavated/explored early in the history of archaeology that became intrinsic for defining the Iron Age period in Britain (c. 800 BC – AD 43). We are currently looking at Swallowcliffe Down in Wiltshire where many Early Iron Age period artefacts were recovered. The following is an excerpt from our project proposal:

The Early Iron Age settlement at Swallowcliffe Down, Wiltshire, was excavated in the 1920s by Dr RCC Clay1,2. He uncovered a rich assemblage of artefacts including pottery, glass beads, and an assortment of other domestic material. Clay’s technique for finding archaeological features relied on striking the ground and listening for changes in the vibrations, which located a total of 93 pits. Today, we would recognise the pits as only one type of feature that is often present at settlements and we would interpret the finds within them as acts of structured deposition3. However, as the study of the British Iron Age was still in its infancy, Clay recognised that only some of the pits were used for storage, while he interpreted most of them erroneously as Iron Age domestic structures. As a consequence of the ambiguity of the deposits, the material found was considered to be solely the result of domestic life. Due to his excavation methodology and the state of knowledge at the time of these excavations, other, smaller features, such as post-holes for roundhouses or 4-post structures, may well be present on the site, but remain unidentified.

The lack of structural evidence at this site renders discussions of settlement impossible; as a consequence the artefactual evidence has been the focus of subsequent study. This has led to some interesting results. Amongst the material found in pits was evidence for iron smelting, bronze working, and pottery manufacture. Interestingly, the artefactual evidence suggests that there was an exceptional level of continental connectivity. For example, although much of the pottery falls within the Early Iron Age types (i.e. bucket with slightly everted rims) that bear similarity to the All Cannings Cross type, two unusual forms have been suggested to be local copies of continental La Tène types4. In addition, the five glass beads have continental parallels. As settlements within Wiltshire have traditionally been seen within the Wessex hillfort-dominated approach, the range of artefacts and the continental affiliations found at Swallowcliffe Down (a non-hillfort site) suggests that the situation is much more complex, which has wider implications for our understanding of social organisation during this period5.

 

More about our project in the next blog post!

 

Bibliography

1. R. C. C. Clay, An Inhabited Site of La Tene I date, on Swallowcliffe Down. Wiltshire Archaeology and Natural History Society 43, 59 (1925).

2. R. C. C. Clay, Supplementary Report on the Early Iron Age Village on Swallowcliffe Down. Wiltshire Archaeology and Natural History Society 43, 540 (1927).

3. J. D. Hill, Ritual and rubbish in the Iron Age of Wessex: a study of the formation of a specific archaeological record.  (BAR British Series no. 242, Oxford, 1995).

4. B. Cunliffe, Iron Age Communities in Britain: An Account of England, Scotland and Wales from the Seventh Century BC until the Roman Conquest, Fourth Edition.  (Routledge, London, 2005).

5. J. D. Hill, in The Iron Age in Britain and Ireland: Recent Trends, T. C. Champion, J. R. Collis, Eds. (J.R. Collis Publications, Sheffield, 1996),  pp. 95-116.